Responses of solitary tract nucleus neurons to taste and mechanical stimulations of the oral cavity in decerebrate rats.

Physiological characteristics of 45 taste and 15 mechanoreceptive units were examined in the solitary tract nucleus (NTS) of rats decerebrated at the pre- or midcollicular level, and compared with previous findings in the intact rat. The rostro-caudal extent of the area, where taste and mechanoreceptive neurons were recorded, was almost the same in the decerebrate rat as that in intact rat. The spontaneous discharge rate was significantly lower in the decerebrate rat than in the intact rat. The taste profile of the NTS units in decerebrate rats was quite different from that in intact rats; significant decreases in correlation coefficients were found between certain pairs of taste stimuli and spontaneous discharge rate, e.g. NaCl-quinine, sucrose-quinine. A large number of taste (18 of 31) and mechanoreceptive (12 of 15) units examined had receptive fields (RFs) on the palate, and four taste and two mechanoreceptive units on the circumvallate area. This contrasts with the findings in the intact rat. Some taste (n = 1) and mechanoreceptive units (n = 2) had large RFs. Taste units with different RF locations showed different taste profiles. Acute i.v. injection of amobarbital sodium affected only the response magnitude of taste units, suggesting that most of the differences between intact and decerebrate rats might be caused by decerebration. The present findings indicate that neural structures above the pre- or midcollicular level have tonic inhibitory or facilitatory effects on the response properties of NTS taste units.